FIG. 11 is an example of prior art disclosed in a U.S. Pat. No. 4,430,864 and comprises: a process air passage A; a regeneration air passage B; two desiccant beds 103A, 103B; and a heat pump device 200 for desiccant regeneration and cooling of process air. The heat pump device 200 is provided with two heat exchangers embedded in the two desiccant beds 103A, 103B, one of the desiccant bed is used as a high/low temperature heat source. One of the desiccant beds is used to flow process air to carry out dehumidification, and the other desiccant bed is used for flowing regeneration air to carry out desiccant regeneration. After these processes have been carried out for sometime, regeneration air and process air are switched by means of switching valves 105, 106 to carry out reverse steps.
In the technology described above, the high/low temperature sources and the desiccant devices are integrated into one unit respectively, and an amount of heat corresponding to the cooling effect .DELTA.Q for the air conditioning system becomes a thermal load on the heat pump (refrigerating machine). The thermal efficiency of the entire system is thus limited by the capacity of the heat pump, and no extra effect is achieved within the system. Therefore, it may be concluded that complexity of the system is not worth the effort.
To resolve such a problem, the following type of arrangement may be considered. That is, as shown in FIG. 12, a high temperature source 220, is disposed in the regeneration air passage to heat the regeneration air, while a low temperature heat source 210 is disposed in the process air passage to cool the process air. Also, a heat exchanger 104 may be provided for transferring sensible heat between post-desiccant process air the pre-desiccant regeneration air. In the example shown, the desiccant device is a desiccant wheel 103 rotatable to traverse the process air passage A and the regeneration air passage B.
In such a system, as shown in a psychrometric chart in FIG. 13, total cooling effect (.DELTA.Q) of a cooling effect produced by the sensible heat exchanger added to the cooling effect (A q) provided by the heat pump device may be obtained for the entire system, thus resulting in a higher thermal efficiency and a more compact design of the entire system than the system shown in FIG. 11.
The heat pump used for this purpose requires a high-temperature heat source of over 65.degree. C. for desiccant desorption and a low-temperature heat source of about 10.degree. C. that for cooling the process air. FIG. 14 shows a Mollier diagram of a refrigerant, HFC134a, in a vapor compression refrigeration cycle operated by such high-temperature and low-temperature sources. As shown in FIG. 14, the amount of temperature rise by the heat pump is 55.degree. C. such that the pressure ratio and the compressor power are almost the same as the heat pump for conventional air conditioners based on the HCFC22 refrigerant, therefore, it may be possible to use a compressor designed to use HCFC22 in the heat pump of a desiccant-assisted air conditioning system. There is a further possibility that, by using the sensible heat of the superheated vapor (80.degree. C. in the diagram) at the compressor outlet, the desiccant regeneration air can be heated to a temperature higher than the condensation temperature.
However, even in such an air conditioning system, heat utilization leaves much room for improvement, because of the relationship shown in FIG. 15 between the changes in the refrigerant, the desiccant regeneration air and enthalpy when all of the regeneration air is passed through the high-temperature heat exchanger of the heat pump shown in FIG. 12. It can be seen from FIG. 15 that, assuming a thermal efficiency of 80% for the transfer of condensation heat in the high-temperature heat exchanger 220, the temperature of regeneration air is raised by about 20.degree. C. from 40 to 60.degree. C. However, the heating ability of superheated vapor in the heat pump is only 12% of the total heating ability of the heat pump, as indicated in FIG. 14, therefore, when the regeneration air is heated with this remaining 12%, the temperature rise that can be expected is only about: EQU (20.degree. C./0.88).times.0.12.times.2.7.degree. C.
The result is that, the sensible heat of the superheated vapor from the compressor outlet can hardly contribute to raising the regeneration air temperature, and the system is forced to carry out desiccant desorption at a temperature (62.7.degree. C. in the diagram) which is lower than the refrigerant condensation temperature. When a desiccant material such as silica gel is used, there is a tendency that the higher the temperature of regeneration air the higher the dehumidification capacity of the regenerated desiccant up to about 90.degree. C. regeneration temperature. Therefore, the higher the temperature of the regeneration air the higher the processing capability of a desiccant-assisted air conditioner for processing latent heat, and the cooling capacity of the system is improved. If it is attempted to raise the condensation temperature of a refrigerant to about 75.degree. C. in order to achieve such a purpose of raising the desiccant desorbing temperature, the refrigeration cycle of the system is disturbed to a dotted line shown in FIG. 14 such that the condensation pressure required becomes abnormally high (24.1 kg/cm.sup.2), and consequently, a compressor designed for HCFC22 can no longer be used as a compressor for a heat pump in a desiccant-assisted air conditioning system and a compressor of a higher compressor power leads to a lower coefficient of performance.